7.1.2 New Results since the SAR

Improved atmospheric and oceanic modules of coupled climate models, especially
improved representation of clouds, parametrizations of boundary layer and ocean
mixing, and increased grid resolution, have helped reduce and often eliminate
the need for flux adjustment in some coupled climate models. This has not reduced
the range of sensitivities in projection experiments.

There is a growing appreciation of the importance of the stratosphere, particularly
the lower stratosphere, in the climate system. Since the mass of the stratosphere
represents only about 10 to 20% of the atmospheric mass, the traditional view
has been that the stratosphere can play only a limited role in climate change.
However, this view is changing. The transport and distribution of radiatively
active constituents, especially water vapour and ozone, are important for radiative
forcing. Moreover, waves generated in the troposphere propagate into the stratosphere
and are absorbed, so that stratospheric changes alter where and how they are
absorbed, and effects can extend downward into the troposphere.

Observational records suggest that the atmosphere may exhibit specific regimes
which characterise the climate on a regional to hemispheric scale. Climate change
may thus manifest itself both as shifting means as well as changing preference
of specific regimes. Examples are the North Atlantic Oscillation (NAO) index,
which shows a bias toward positive values for the last 30 years, and the climate
"shift" in the tropical Pacific at around 1976.

While considerable advances have been made in improving feedbacks and coupled
processes and their depiction in models, the emergence of the role of natural
modes of the climate system such as the El Niño-Southern Oscillation
(ENSO) and NAO as key determinants of regional climate change, and possibly
also shifts, has led to an increase in uncertainty in those aspects of climate
change that critically depend on regional changes. It is encouraging that the
most advanced models exhibit natural variability that resembles the most important
modes such as ENSO and NAO.

The coupled ocean-atmosphere system contains important non-linearities which
give rise to a multiplicity of states of the Atlantic thermohaline circulation
(THC). Most climate models respond to global warming by a reduction of the Atlantic
THC. A complete shut-down of the THC in response to continued warming cannot
be excluded and would occur if certain thresholds are crossed. Models have identified
the maximum strength of greenhouse gas induced forcing and the rate of increase
as thresholds for the maintenance of the THC in the Atlantic ocean, an important
process influencing the climate of the Northern Hemisphere. While such thresholds
have been found in a variety of fundamentally different models, suggesting that
their existence in the climate system is a robust result, we cannot yet determine
with accuracy the values of these thresholds, because they crucially depend
on the response of the atmospheric hydrological cycle to climate change.

The representation of sea-ice dynamics and sub-grid scale processes in coupled
models has improved significantly, which is an important prerequisite for a
better understanding of, the current variability in, and a more accurate prediction
of future changes in polar sea-ice cover and atmosphere-ocean interaction in
areas of deep water formation.

Recent model simulations, including new land-surface parametrizations and field
observations, strongly indicate that large-scale changes in land use can lead
to significant impacts on the regional climate. The terrestrial carbon and water
cycles are also linked through vegetation physiology, which regulates the ratio
of carbon dioxide (CO2) uptake (photosynthesis) to water loss (evapotranspiration).
As a result, vegetation water-use efficiency is likely to change with increasing
atmospheric CO2, leading to a reduction in evapotranspiration over densely vegetated
areas. Tropical deforestation, in particular, is associated with local warming
and drying. However, realistic land-use change scenarios for the next 50 to
100 years are not expected to give rise to global scale climate changes comparable
to those resulting from greenhouse gas warming.